In a tankyard where a plurality of tanks for storing liquid such as petroleum or the like are installed, it is necessary to monitor the liquid level during the operation of pouring the liquid into each tank or of allowing the liquid to flow out. In order to determine the net quantity of the liquid within the tank, it is also necessary to detect the liquid temperature and to convert such temperature into a compensation value based on a standard temperature, in addition to the detection of the liquid level, since the liquid volume within the tank varies with changes in temperature. Monitoring of the temperature and the level of the liquid contained in a number of tanks could be attained by installing a measuring instrument in each tank and by having a man to go to each tank for inspection. Such an inspection method, however, will require laborious work and hence not be efficient. It will thus be necessary to provide a system by means of which the liquid volume in each tank can be centrally monitored and controlled.